(1) Field of the Invention
This invention relates to pipe-in-pipe or ‘PiP’ structures suitable for subsea applications.
PiP structures are used, for example, in the offshore oil and gas industry as flowlines to transport hydrocarbons across the seabed and from the seabed toward the surface. They may also be used as flowlines to transport other fluids such as hot water.
PiP structures comprise an inner pipe and an outer pipe in concentric relation, spaced from each other along most of their length to define an annulus between them. They provide a high degree of thermal insulation because the annulus typically contains an insulating material and/or is evacuated to restrict heat transfer between the inner pipe and the outer pipe. They also enhance mechanical strength and leak protection by virtue of their double-walled construction.
The most common purpose of insulation in a subsea PiP structure is to retain heat in hydrocarbons or other hot fluids flowing within the inner pipe serving as the flowline, by resisting heat transfer from those fluids to the much colder water surrounding the outer pipe. However, subsea cryogenic pipelines are also known for the transportation of liquefied gases such as LNG; in that case, PiP structures may be used to resist heat transfer from the water surrounding the outer pipe to the fluid flowing at very low temperatures within the inner pipe.
PiP structures are known to be suitable for offshore fabrication on, and laying from, a pipelaying vessel using J-lay or S-lay techniques, the latter including Steep S-lay. In those techniques, PiP sections or ‘pipe joints’ are welded successively at field joints to an upper end of a pipe string extending from a hang-off structure of the vessel toward the seabed. The welds are tested and the field joints are coated before each section of the pipe string is lowered into the sea. The repetitive welding, testing and coating operations lie on the critical path and so influence the laying rate.
(2) Description of Related Art
WO 2008/053251 to Acergy discloses PiP sections suitable for onshore prefabrication and explains how they may be used in J-lay operations as an example. Each PiP section comprises an inner pipe section that protrudes at each end from a shorter outer pipe section, with the inset ends of the outer pipe section being swaged conically toward and welded to the inner pipe section to close the annulus. This leaves ends of the inner pipe section protruding from the outer pipe section. Once closed, the annulus can be evacuated during the prefabrication process, removing that operation from the critical path during subsequent offshore operations.
U.S. Pat. No. 6,446,321 to ITP is referenced in WO 2008/053251 as an example of a swaging technique.
When one of the PiP sections of WO 2008/053251 is to be added to the pipe string on a pipelaying vessel, protruding ends of abutting inner pipe sections are butt-welded together to form a field joint in the flowline. Once the weld is tested, an insulating and protective sleeve is slid along an outer pipe section into alignment with the field joint to be fixed to the outside of the outer pipe sections around the field joint. Finally, the void around the field joint between the sleeve, the protruding ends of the inner pipe sections and the swaged ends of the outer pipe sections is filled with a resin.
In contrast to J-lay or S-lay techniques, reel-lay operations involve winding or spooling a continuous pipe of welded elements onto a reel carried by a pipelaying vessel, to be unwound or unspooled and straightened during pipelaying at sea. Fabrication and winding of the pipe typically takes place at a spoolbase that the vessel visits when necessary for loading.
At the spoolbase, multiple standard pipe joints are welded together to form very long stalks, which may for example be 750 m or more in length. Efficiently, stalks can be assembled and stored while a pipelaying vessel is away from the spoolbase on pipelaying operations. When a pipelaying vessel visits the spoolbase to be loaded, the stalks are welded together successively end-to-end to create a continuous length of pipe as the pipe is being wound onto the reel of the vessel.
Known swaged PiP connection arrangements disclosed in WO 2008/053251 and U.S. Pat. No. 6,446,321 are not suitable for reel-lay operations because the sleeve protruding radially from the outer pipe creates a significant over-thickness that precludes reeling after welding. This denies the advantages of swaged PiP solutions to reel-lay applications.
GB 2453438 relates to thermal insulation of a screw-threaded junction between PiP sections. GB 2191842 discloses an end wall arrangement for PiP sections that also involves screw threads. No concern for reelability is apparent in either document.
WO 2009/085051 discloses a coupling system but this is for lined pipe sections rather than for PiP sections.
There remains a need for a reelable PiP solution with a pressure-tight barrier that allows a simple fabrication process while maintaining thermal insulation based on a reduced pressure within the annulus. Such a durable, easy-to-fabricate and reelable leak-tight solution did not exist before the present invention.
It is against this background that the present invention has been devised.